The reliability of electronic systems can suffer due to the presence of latent open defects in the circuit boards or ceramic modules of electronic packages. Such defects consist of a local reduction in cross sections which may result from breakages, narrow conductors, intermittent opens, etc. in the cross sectional area of the conducting lines of the package. The reduced cross section leads to an increased possibility that an open circuit will develop as a result of applied stresses from processing, assembly, or use. Typically, the excess resistance of such a defect cannot be detected directly because it is much smaller than the resistance differences arising from normal process variations.
Two test systems which are capable of detecting latent open defects are in use today. These are Electric Board Tester (hereafter referred to as "EBT") which is used for detecting defects on epoxy-glass circuit boards, and the Electric Module Tester (hereafter referred to as "EMT"), which is used to test the interconnection circuitry on multi-layer ceramic modules. One EBT is illustrated in U.S. Pat. No. 4,496,900 entitled "Nonlinearity Detection Using Fault-Generated Second Harmonic" issued Jan. 29, 1985 to DiStefano et al. (incorporated herein by reference). One EMT is illustrated in U.S. Pat. No. 4,868,506 entitled "Defect Detection Using Intermodulation Signals" issued Sep. 19, 1989 to DiStefano et al. (incorporated herein by reference). Both of the above EMT and EBT patents are assigned to the assignee of the present invention, and involve the detection of nonlinear electrical effects arising from the resistance modulation produced by local heating at the defect site. The present invention relies on a similar physical mechanism, but uses a different type of excitation and detection technique as described below. The following describes each version in more detail, and points out distinctions between the present invention and both existing testers.
In the EBT system the magnitude of the drive current that is passed through the circuit line under test is given by: EQU I=I.sub.DC +I.sub.1 sin (.omega.t). (1)
The nonlinear electrical effect at a defect results in the generation of harmonics of the drive frequency, and the EBT operates by measuring the amplitude and phase of the second harmonic voltage appearing across the circuit line. The first harmonic is related to the characteristics of the conductor itself, but certain of the higher order harmonics result from the defect and are capable of providing significant information about the characteristics of the defect. More specifically, a power, P=I.sup.2 R.sub.d, is dissipated at a local defect having resistance R.sub.d. A local temperature has an AC response to this heat input which is given by: ##EQU1## where .beta.(.omega.) and .theta.(.omega.) represent the magnitudes and phases, respectively, of the thermal response to the electric drive currents cycling at the frequency .omega.. These terms depend upon the details of the local thermal environment of the conductor, which may be analyzed to provide considerable useful information about the defect. A similar effect occurs in a distributed fashion along a length of circuit lines with a lack of defects (referred to herein as "good-lines"), which gives rise to an overall temperature modulation having the same form as equation 2. The thermal environment for a long section of good-line differs dramatically from that of a localized defect, so the magnitudes and (more importantly for the present application) the phases of their response are quite different. By detecting voltages at a precise phase, chosen to be orthogonal to the phase of the good-line signal, the EBT achieves excellent selectivity for defect signals even for very long circuit lines. Any portion of the signal which is parallel to the good-line signal is considered as relating only to the good-line signal; while any portion of the signal which is orthogonal to the good-line signal relates only to the defect signal.
The temperature modulation gives rise to resistance changes equal to z.alpha.R.DELTA.T, where .alpha. is the temperature coefficient of resistivity. The nonlinear voltage signal across the circuit line, V.sub.d, equals the drive current times this resistance. The second harmonic term is given by: ##EQU2##
It turns out that for defects of interest this signal has an amplitude as small as a fraction of a microvolt. This defect signal must be detected in the presence of a fundamental signal of the conductor which has an amplitude which is nearly seven orders of magnitude larger than the defect signal. To achieve this defect signal recognition, a passive filter is used to reduce the magnitude of the fundamental signal prior to a preamplifier, and the amplifier itself must provide extremely low levels of harmonic distortion. These requirements lead to further associated expenses. Furthermore, the current source must have extraordinarily small levels of harmonic content (1 in 10.sup.7). This limits the drive current to about 0.6 amps, and the frequency to about 1 KHz.
Several tuned circuits are needed in EBTs, and the system is thereby limited to only a single frequency of operation. The EBT works well for circuit lines on epoxy-glass boards, and more generally for testing defects in conductors when the conductors have relatively large cross section. For very thin conductors the size of the good-line signal becomes so large that the use of phase selectivity is insufficient to allow discrimination resulting from the defect signal compared to normal variations in good-line signals. In such cases, the EBT system may have limited output response characteristics. The amplitude of the good-line signals drops off inversely with frequency while the defect signals tend to have a constant amplitude versus frequency so it is very advantageous to work at higher frequencies.
The EMT system improves the testability of conductors having a relatively thin cross-section by using a much higher operating frequency. Since harmonic distortion in amplifiers increases rapidly with frequency, a different scheme than simple frequency doubling was employed in EMTs. In this system two separate frequencies are used in addition to DC, and the signal of interest which is used to detect defects occurs at the difference frequency. The drive current is given by: EQU I=I.sub.DC +I.sub.1 sin ( .omega..sub.1 t)+I.sub.2 sin (.omega..sub.2 t).(4)
Power is dissipated as the square of the current, giving rise to resistance modulation at frequencies .omega..sub.1, .omega..sub.2, 2.omega..sub.1, 2.omega..sub.2, .omega..sub.2 -.omega..sub.1, and .omega..sub.2 +.omega..sub.1. Multiplying by the current to get the voltage and keeping only the difference frequency terms yields: ##EQU3##
The four terms relate to temperature modulation at .omega..sub.1, .omega..sub.2, and .omega..sub.2 -.omega..sub.1, and contain their respective amplitude factors and phases. The ability to distinguish signals from conductor with defects from good-line signals (which are considered as those signals produced by the conductor itself in the absence of any defects) by means of the phase is accomplished in the same manner as in the EBT. The two frequencies are generated by separate circuits and summed using passive components to minimize intermodulation distortion that would introduce additional .omega..sub.2 -.omega..sub.1 component in the drive current. Tuned circuits are required in the drive circuits to further reduce the coupling between the two oscillators. The detector circuit also contains tuned filters to reject the relatively enormous signals present at the fundamental frequencies. The frequencies are approximately 1.5 MHz and 1.0 MHz, giving rise to a difference frequency signal of about 0.5 MHz. These frequencies are very well suited to detecting defects in circuit lines and such lines as those typically found in ceramic modules. The drive currents are limited to about 0.5 amps and the system operates with fixed drive frequencies.
A drawback of operating at very high frequencies in EMTs is that only defects having a relatively short length within a conductor provide the best defect testing characteristics. The maximum detectable defect length and operating frequency are related because of the phase selectivity that is used to allow detection of defects in the presence of large good-line signals. For a very short defect, the surrounding sections of good-line act as a thermal reservoir connected to the defect region by a low thermal impedance (the connection is via metal which has a low thermal resistance). The temperature of the localized defective region therefore tracks the applied AC power virtually directly and instantaneously, with very little phase shift. The magnitude of the temperature swing is independent of frequency in conductors with short defects. A long section of good-line, however, has a much higher heat capacity than a small defect and is only weakly coupled to its surroundings. Thus with long sections of good-lines, the temperature loosely tracks the time integral of the AC power resulting in a phase shift of approximately 90 degrees. Also, the magnitude of the temperature swing is inversely proportional to frequency because of the integration effect. For defects of finite length, the thermal response varies between these two limiting behaviors, depending on the operating frequency. Roughly speaking, the heat capacity of a defective region times the thermal impedance between it and the nearest thermal reservoir yields a thermal response time. If the frequency of the AC power dissipation is relatively high, then the defect response signal may be indistinguishable from the good-line signal.
It should be noted that the phase of the thermal response of actual circuit lines is not exactly equal to 90 degrees, even if there are no defects present. Details of the line width, thickness, composition, and environment make real systems more complicated than the simple model described above. Phase shifts of as much as 35 degrees from the ideal model of 90 degrees have been observed. This is why the ability to finely adjust the detection phase is important in defect detection applications. Each product type, and even different line types within a given product, may have a characteristic phase for good-lines. To achieve a sufficient detectability between defect signal and good-line signals, it is necessary to adjust and maintain the detection phase within 0.6 degrees of orthogonality from the good-line phase. This becomes evident when one realizes that the output for the good-line signals are sometimes 100 times as large as the defect signals.
The optimum frequency of operation depends on a trade-off between the desire to minimize the size of the good-line signal by going to high frequencies (in EMT) against the need to detect defects that may be relatively long by operating at low frequencies (in EBT). Thus, the ability to operate over a wide range of frequencies with one instrument would offer a significant advantage in flexibility. Furthermore, combining a series of measurements made at various frequencies allows one to determine the frequency at which the behavior of the defect crosses over between the two limits, thereby providing additional information concerning the length of a defect. This can be valuable in screening tests since a given defect signal could arise either from a short, severe constriction or a long, slight one, with the latter typically being much less of a reliability risk during operation of the circuit.
The sensitivity of the EBT and EMT scales as the cube of the drive current, so even modest increases in current can yield substantial gains in sensitivity. The present levels of sensitivity are thought to be sufficient to meet the reliability needs of existing products, so lowering the threshold of detection by increasing sensitivity is not required. However, the speed at which a test can be performed can be increased if the signal levels are raised. For example, an increase of a factor of two in drive currents would yield an increase by a multiple of eight in signal, which could be traded off for an increase in detector bandwidth of a factor of 64. The EBT system requires several tenths of a second to perform a measurement, and reducing this time could substantially improve overall throughput in a typical robotic probing system. The EMT runs about ten times faster than the EBT so it has less impact on total probe time, but faster performance would greatly impact the test time for relay switched, bed-of-nails testing. The present invention circumvents the key limiting factors of both the EBT and EMT, allowing the use of higher operating currents and a wide range of frequencies in a single system.
Another defect detector is illustrated in U.S. Pat. No. 3,500,188, entitled "Method and Means For Measuring Constriction Resistance Based on Nonlinearity" issued Mar. 10, 1970 to Whitley (incorporated herein by reference). This U.S. Patent illustrates the use of currents of two frequencies 1f and 2f, and observing a DC voltage produced thereby. Whitley described the use of a detection frequency which is far from either of the excitation frequencies. This detection frequency can be used to detect the characteristics, and presence, of a constriction resistance. Whitley's system is non-destructive. In Whitley's system, there is no phase selectivity described. There is no description of any connection between the two driving frequency signals 1f and 2f and the thermal response, which is one very desirable feature of the present invention. Only a relatively crude defect detection may be performed without these features.
Another system is capable of both detecting and locating latent defects exists. This system is limited to use on exposed levels of circuitry, and operates by utilizing eddy currents. This system operates in a non-destructive manner.